1.\" 2.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 3.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 4.\" All rights reserved 5.\" 6.\" As far as I am concerned, the code I have written for this software 7.\" can be used freely for any purpose. Any derived versions of this 8.\" software must be clearly marked as such, and if the derived work is 9.\" incompatible with the protocol description in the RFC file, it must be 10.\" called by a name other than "ssh" or "Secure Shell". 11.\" 12.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 13.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 14.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 15.\" 16.\" Redistribution and use in source and binary forms, with or without 17.\" modification, are permitted provided that the following conditions 18.\" are met: 19.\" 1. Redistributions of source code must retain the above copyright 20.\" notice, this list of conditions and the following disclaimer. 21.\" 2. Redistributions in binary form must reproduce the above copyright 22.\" notice, this list of conditions and the following disclaimer in the 23.\" documentation and/or other materials provided with the distribution. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 26.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 27.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 28.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 29.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 30.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 31.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 32.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 33.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 34.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 35.\" 36.\" $OpenBSD: ssh_config.5,v 1.146 2010/12/08 04:02:47 djm Exp $ 37.\" $FreeBSD$ 38.Dd December 8, 2010 39.Dt SSH_CONFIG 5 40.Os 41.Sh NAME 42.Nm ssh_config 43.Nd OpenSSH SSH client configuration files 44.Sh SYNOPSIS 45.Nm ~/.ssh/config 46.Nm /etc/ssh/ssh_config 47.Sh DESCRIPTION 48.Xr ssh 1 49obtains configuration data from the following sources in 50the following order: 51.Pp 52.Bl -enum -offset indent -compact 53.It 54command-line options 55.It 56user's configuration file 57.Pq Pa ~/.ssh/config 58.It 59system-wide configuration file 60.Pq Pa /etc/ssh/ssh_config 61.El 62.Pp 63For each parameter, the first obtained value 64will be used. 65The configuration files contain sections separated by 66.Dq Host 67specifications, and that section is only applied for hosts that 68match one of the patterns given in the specification. 69The matched host name is the one given on the command line. 70.Pp 71Since the first obtained value for each parameter is used, more 72host-specific declarations should be given near the beginning of the 73file, and general defaults at the end. 74.Pp 75The configuration file has the following format: 76.Pp 77Empty lines and lines starting with 78.Ql # 79are comments. 80Otherwise a line is of the format 81.Dq keyword arguments . 82Configuration options may be separated by whitespace or 83optional whitespace and exactly one 84.Ql = ; 85the latter format is useful to avoid the need to quote whitespace 86when specifying configuration options using the 87.Nm ssh , 88.Nm scp , 89and 90.Nm sftp 91.Fl o 92option. 93Arguments may optionally be enclosed in double quotes 94.Pq \&" 95in order to represent arguments containing spaces. 96.Pp 97The possible 98keywords and their meanings are as follows (note that 99keywords are case-insensitive and arguments are case-sensitive): 100.Bl -tag -width Ds 101.It Cm Host 102Restricts the following declarations (up to the next 103.Cm Host 104keyword) to be only for those hosts that match one of the patterns 105given after the keyword. 106If more than one pattern is provided, they should be separated by whitespace. 107A single 108.Ql * 109as a pattern can be used to provide global 110defaults for all hosts. 111The host is the 112.Ar hostname 113argument given on the command line (i.e. the name is not converted to 114a canonicalized host name before matching). 115.Pp 116See 117.Sx PATTERNS 118for more information on patterns. 119.It Cm AddressFamily 120Specifies which address family to use when connecting. 121Valid arguments are 122.Dq any , 123.Dq inet 124(use IPv4 only), or 125.Dq inet6 126(use IPv6 only). 127.It Cm BatchMode 128If set to 129.Dq yes , 130passphrase/password querying will be disabled. 131This option is useful in scripts and other batch jobs where no user 132is present to supply the password. 133The argument must be 134.Dq yes 135or 136.Dq no . 137The default is 138.Dq no . 139.It Cm BindAddress 140Use the specified address on the local machine as the source address of 141the connection. 142Only useful on systems with more than one address. 143Note that this option does not work if 144.Cm UsePrivilegedPort 145is set to 146.Dq yes . 147.It Cm ChallengeResponseAuthentication 148Specifies whether to use challenge-response authentication. 149The argument to this keyword must be 150.Dq yes 151or 152.Dq no . 153The default is 154.Dq yes . 155.It Cm CheckHostIP 156If this flag is set to 157.Dq yes , 158.Xr ssh 1 159will additionally check the host IP address in the 160.Pa known_hosts 161file. 162This allows ssh to detect if a host key changed due to DNS spoofing. 163If the option is set to 164.Dq no , 165the check will not be executed. 166The default is 167.Dq no . 168.It Cm Cipher 169Specifies the cipher to use for encrypting the session 170in protocol version 1. 171Currently, 172.Dq blowfish , 173.Dq 3des , 174and 175.Dq des 176are supported. 177.Ar des 178is only supported in the 179.Xr ssh 1 180client for interoperability with legacy protocol 1 implementations 181that do not support the 182.Ar 3des 183cipher. 184Its use is strongly discouraged due to cryptographic weaknesses. 185The default is 186.Dq 3des . 187.It Cm Ciphers 188Specifies the ciphers allowed for protocol version 2 189in order of preference. 190Multiple ciphers must be comma-separated. 191The supported ciphers are 192.Dq 3des-cbc , 193.Dq aes128-cbc , 194.Dq aes192-cbc , 195.Dq aes256-cbc , 196.Dq aes128-ctr , 197.Dq aes192-ctr , 198.Dq aes256-ctr , 199.Dq arcfour128 , 200.Dq arcfour256 , 201.Dq arcfour , 202.Dq blowfish-cbc , 203and 204.Dq cast128-cbc . 205The default is: 206.Bd -literal -offset 3n 207aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 208aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 209aes256-cbc,arcfour 210.Ed 211.It Cm ClearAllForwardings 212Specifies that all local, remote, and dynamic port forwardings 213specified in the configuration files or on the command line be 214cleared. 215This option is primarily useful when used from the 216.Xr ssh 1 217command line to clear port forwardings set in 218configuration files, and is automatically set by 219.Xr scp 1 220and 221.Xr sftp 1 . 222The argument must be 223.Dq yes 224or 225.Dq no . 226The default is 227.Dq no . 228.It Cm Compression 229Specifies whether to use compression. 230The argument must be 231.Dq yes 232or 233.Dq no . 234The default is 235.Dq no . 236.It Cm CompressionLevel 237Specifies the compression level to use if compression is enabled. 238The argument must be an integer from 1 (fast) to 9 (slow, best). 239The default level is 6, which is good for most applications. 240The meaning of the values is the same as in 241.Xr gzip 1 . 242Note that this option applies to protocol version 1 only. 243.It Cm ConnectionAttempts 244Specifies the number of tries (one per second) to make before exiting. 245The argument must be an integer. 246This may be useful in scripts if the connection sometimes fails. 247The default is 1. 248.It Cm ConnectTimeout 249Specifies the timeout (in seconds) used when connecting to the 250SSH server, instead of using the default system TCP timeout. 251This value is used only when the target is down or really unreachable, 252not when it refuses the connection. 253.It Cm ControlMaster 254Enables the sharing of multiple sessions over a single network connection. 255When set to 256.Dq yes , 257.Xr ssh 1 258will listen for connections on a control socket specified using the 259.Cm ControlPath 260argument. 261Additional sessions can connect to this socket using the same 262.Cm ControlPath 263with 264.Cm ControlMaster 265set to 266.Dq no 267(the default). 268These sessions will try to reuse the master instance's network connection 269rather than initiating new ones, but will fall back to connecting normally 270if the control socket does not exist, or is not listening. 271.Pp 272Setting this to 273.Dq ask 274will cause ssh 275to listen for control connections, but require confirmation using the 276.Ev SSH_ASKPASS 277program before they are accepted (see 278.Xr ssh-add 1 279for details). 280If the 281.Cm ControlPath 282cannot be opened, 283ssh will continue without connecting to a master instance. 284.Pp 285X11 and 286.Xr ssh-agent 1 287forwarding is supported over these multiplexed connections, however the 288display and agent forwarded will be the one belonging to the master 289connection i.e. it is not possible to forward multiple displays or agents. 290.Pp 291Two additional options allow for opportunistic multiplexing: try to use a 292master connection but fall back to creating a new one if one does not already 293exist. 294These options are: 295.Dq auto 296and 297.Dq autoask . 298The latter requires confirmation like the 299.Dq ask 300option. 301.It Cm ControlPath 302Specify the path to the control socket used for connection sharing as described 303in the 304.Cm ControlMaster 305section above or the string 306.Dq none 307to disable connection sharing. 308In the path, 309.Ql %l 310will be substituted by the local host name, 311.Ql %h 312will be substituted by the target host name, 313.Ql %p 314the port, and 315.Ql %r 316by the remote login username. 317It is recommended that any 318.Cm ControlPath 319used for opportunistic connection sharing include 320at least %h, %p, and %r. 321This ensures that shared connections are uniquely identified. 322.It Cm ControlPersist 323When used in conjunction with 324.Cm ControlMaster , 325specifies that the master connection should remain open 326in the background (waiting for future client connections) 327after the initial client connection has been closed. 328If set to 329.Dq no , 330then the master connection will not be placed into the background, 331and will close as soon as the initial client connection is closed. 332If set to 333.Dq yes , 334then the master connection will remain in the background indefinitely 335(until killed or closed via a mechanism such as the 336.Xr ssh 1 337.Dq Fl O No exit 338option). 339If set to a time in seconds, or a time in any of the formats documented in 340.Xr sshd_config 5 , 341then the backgrounded master connection will automatically terminate 342after it has remained idle (with no client connections) for the 343specified time. 344.It Cm DynamicForward 345Specifies that a TCP port on the local machine be forwarded 346over the secure channel, and the application 347protocol is then used to determine where to connect to from the 348remote machine. 349.Pp 350The argument must be 351.Sm off 352.Oo Ar bind_address : Oc Ar port . 353.Sm on 354IPv6 addresses can be specified by enclosing addresses in square brackets. 355By default, the local port is bound in accordance with the 356.Cm GatewayPorts 357setting. 358However, an explicit 359.Ar bind_address 360may be used to bind the connection to a specific address. 361The 362.Ar bind_address 363of 364.Dq localhost 365indicates that the listening port be bound for local use only, while an 366empty address or 367.Sq * 368indicates that the port should be available from all interfaces. 369.Pp 370Currently the SOCKS4 and SOCKS5 protocols are supported, and 371.Xr ssh 1 372will act as a SOCKS server. 373Multiple forwardings may be specified, and 374additional forwardings can be given on the command line. 375Only the superuser can forward privileged ports. 376.It Cm EnableSSHKeysign 377Setting this option to 378.Dq yes 379in the global client configuration file 380.Pa /etc/ssh/ssh_config 381enables the use of the helper program 382.Xr ssh-keysign 8 383during 384.Cm HostbasedAuthentication . 385The argument must be 386.Dq yes 387or 388.Dq no . 389The default is 390.Dq no . 391This option should be placed in the non-hostspecific section. 392See 393.Xr ssh-keysign 8 394for more information. 395.It Cm EscapeChar 396Sets the escape character (default: 397.Ql ~ ) . 398The escape character can also 399be set on the command line. 400The argument should be a single character, 401.Ql ^ 402followed by a letter, or 403.Dq none 404to disable the escape 405character entirely (making the connection transparent for binary 406data). 407.It Cm ExitOnForwardFailure 408Specifies whether 409.Xr ssh 1 410should terminate the connection if it cannot set up all requested 411dynamic, tunnel, local, and remote port forwardings. 412The argument must be 413.Dq yes 414or 415.Dq no . 416The default is 417.Dq no . 418.It Cm ForwardAgent 419Specifies whether the connection to the authentication agent (if any) 420will be forwarded to the remote machine. 421The argument must be 422.Dq yes 423or 424.Dq no . 425The default is 426.Dq no . 427.Pp 428Agent forwarding should be enabled with caution. 429Users with the ability to bypass file permissions on the remote host 430(for the agent's Unix-domain socket) 431can access the local agent through the forwarded connection. 432An attacker cannot obtain key material from the agent, 433however they can perform operations on the keys that enable them to 434authenticate using the identities loaded into the agent. 435.It Cm ForwardX11 436Specifies whether X11 connections will be automatically redirected 437over the secure channel and 438.Ev DISPLAY 439set. 440The argument must be 441.Dq yes 442or 443.Dq no . 444The default is 445.Dq no . 446.Pp 447X11 forwarding should be enabled with caution. 448Users with the ability to bypass file permissions on the remote host 449(for the user's X11 authorization database) 450can access the local X11 display through the forwarded connection. 451An attacker may then be able to perform activities such as keystroke monitoring 452if the 453.Cm ForwardX11Trusted 454option is also enabled. 455.It Cm ForwardX11Timeout 456Specify a timeout for untrusted X11 forwarding 457using the format described in the 458.Sx TIME FORMATS 459section of 460.Xr sshd_config 5 . 461X11 connections received by 462.Xr ssh 1 463after this time will be refused. 464The default is to disable untrusted X11 forwarding after twenty minutes has 465elapsed. 466.It Cm ForwardX11Trusted 467If this option is set to 468.Dq yes , 469remote X11 clients will have full access to the original X11 display. 470.Pp 471If this option is set to 472.Dq no , 473remote X11 clients will be considered untrusted and prevented 474from stealing or tampering with data belonging to trusted X11 475clients. 476Furthermore, the 477.Xr xauth 1 478token used for the session will be set to expire after 20 minutes. 479Remote clients will be refused access after this time. 480.Pp 481The default is 482.Dq no . 483.Pp 484See the X11 SECURITY extension specification for full details on 485the restrictions imposed on untrusted clients. 486.It Cm GatewayPorts 487Specifies whether remote hosts are allowed to connect to local 488forwarded ports. 489By default, 490.Xr ssh 1 491binds local port forwardings to the loopback address. 492This prevents other remote hosts from connecting to forwarded ports. 493.Cm GatewayPorts 494can be used to specify that ssh 495should bind local port forwardings to the wildcard address, 496thus allowing remote hosts to connect to forwarded ports. 497The argument must be 498.Dq yes 499or 500.Dq no . 501The default is 502.Dq no . 503.It Cm GlobalKnownHostsFile 504Specifies a file to use for the global 505host key database instead of 506.Pa /etc/ssh/ssh_known_hosts . 507.It Cm GSSAPIAuthentication 508Specifies whether user authentication based on GSSAPI is allowed. 509The default is 510.Dq no . 511Note that this option applies to protocol version 2 only. 512.It Cm GSSAPIDelegateCredentials 513Forward (delegate) credentials to the server. 514The default is 515.Dq no . 516Note that this option applies to protocol version 2 only. 517.It Cm HashKnownHosts 518Indicates that 519.Xr ssh 1 520should hash host names and addresses when they are added to 521.Pa ~/.ssh/known_hosts . 522These hashed names may be used normally by 523.Xr ssh 1 524and 525.Xr sshd 8 , 526but they do not reveal identifying information should the file's contents 527be disclosed. 528The default is 529.Dq no . 530Note that existing names and addresses in known hosts files 531will not be converted automatically, 532but may be manually hashed using 533.Xr ssh-keygen 1 . 534.It Cm HostbasedAuthentication 535Specifies whether to try rhosts based authentication with public key 536authentication. 537The argument must be 538.Dq yes 539or 540.Dq no . 541The default is 542.Dq no . 543This option applies to protocol version 2 only and 544is similar to 545.Cm RhostsRSAAuthentication . 546.It Cm HostKeyAlgorithms 547Specifies the protocol version 2 host key algorithms 548that the client wants to use in order of preference. 549The default for this option is: 550.Bd -literal -offset 3n 551ecdsa-sha2-nistp256-cert-v01@openssh.com, 552ecdsa-sha2-nistp384-cert-v01@openssh.com, 553ecdsa-sha2-nistp521-cert-v01@openssh.com, 554ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com, 555ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com, 556ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 557ssh-rsa,ssh-dss 558.Ed 559.Pp 560If hostkeys are known for the destination host then this default is modified 561to prefer their algorithms. 562.It Cm HostKeyAlias 563Specifies an alias that should be used instead of the 564real host name when looking up or saving the host key 565in the host key database files. 566This option is useful for tunneling SSH connections 567or for multiple servers running on a single host. 568.It Cm HostName 569Specifies the real host name to log into. 570This can be used to specify nicknames or abbreviations for hosts. 571If the hostname contains the character sequence 572.Ql %h , 573then this will be replaced with the host name specified on the commandline 574(this is useful for manipulating unqualified names). 575The default is the name given on the command line. 576Numeric IP addresses are also permitted (both on the command line and in 577.Cm HostName 578specifications). 579.It Cm IdentitiesOnly 580Specifies that 581.Xr ssh 1 582should only use the authentication identity files configured in the 583.Nm 584files, 585even if 586.Xr ssh-agent 1 587offers more identities. 588The argument to this keyword must be 589.Dq yes 590or 591.Dq no . 592This option is intended for situations where ssh-agent 593offers many different identities. 594The default is 595.Dq no . 596.It Cm IdentityFile 597Specifies a file from which the user's DSA, ECDSA or DSA authentication 598identity is read. 599The default is 600.Pa ~/.ssh/identity 601for protocol version 1, and 602.Pa ~/.ssh/id_dsa , 603.Pa ~/.ssh/id_ecdsa 604and 605.Pa ~/.ssh/id_rsa 606for protocol version 2. 607Additionally, any identities represented by the authentication agent 608will be used for authentication. 609.Xr ssh 1 610will try to load certificate information from the filename obtained by 611appending 612.Pa -cert.pub 613to the path of a specified 614.Cm IdentityFile . 615.Pp 616The file name may use the tilde 617syntax to refer to a user's home directory or one of the following 618escape characters: 619.Ql %d 620(local user's home directory), 621.Ql %u 622(local user name), 623.Ql %l 624(local host name), 625.Ql %h 626(remote host name) or 627.Ql %r 628(remote user name). 629.Pp 630It is possible to have 631multiple identity files specified in configuration files; all these 632identities will be tried in sequence. 633.It Cm IPQoS 634Specifies the IPv4 type-of-service or DSCP class for connections. 635Accepted values are 636.Dq af11 , 637.Dq af12 , 638.Dq af13 , 639.Dq af14 , 640.Dq af22 , 641.Dq af23 , 642.Dq af31 , 643.Dq af32 , 644.Dq af33 , 645.Dq af41 , 646.Dq af42 , 647.Dq af43 , 648.Dq cs0 , 649.Dq cs1 , 650.Dq cs2 , 651.Dq cs3 , 652.Dq cs4 , 653.Dq cs5 , 654.Dq cs6 , 655.Dq cs7 , 656.Dq ef , 657.Dq lowdelay , 658.Dq throughput , 659.Dq reliability , 660or a numeric value. 661This option may take one or two arguments, separated by whitespace. 662If one argument is specified, it is used as the packet class unconditionally. 663If two values are specified, the first is automatically selected for 664interactive sessions and the second for non-interactive sessions. 665The default is 666.Dq lowdelay 667for interactive sessions and 668.Dq throughput 669for non-interactive sessions. 670.It Cm KbdInteractiveAuthentication 671Specifies whether to use keyboard-interactive authentication. 672The argument to this keyword must be 673.Dq yes 674or 675.Dq no . 676The default is 677.Dq yes . 678.It Cm KbdInteractiveDevices 679Specifies the list of methods to use in keyboard-interactive authentication. 680Multiple method names must be comma-separated. 681The default is to use the server specified list. 682The methods available vary depending on what the server supports. 683For an OpenSSH server, 684it may be zero or more of: 685.Dq bsdauth , 686.Dq pam , 687and 688.Dq skey . 689.It Cm KexAlgorithms 690Specifies the available KEX (Key Exchange) algorithms. 691Multiple algorithms must be comma-separated. 692The default is: 693.Bd -literal -offset indent 694ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 695diffie-hellman-group-exchange-sha256, 696diffie-hellman-group-exchange-sha1, 697diffie-hellman-group14-sha1, 698diffie-hellman-group1-sha1 699.Ed 700.It Cm LocalCommand 701Specifies a command to execute on the local machine after successfully 702connecting to the server. 703The command string extends to the end of the line, and is executed with 704the user's shell. 705The following escape character substitutions will be performed: 706.Ql %d 707(local user's home directory), 708.Ql %h 709(remote host name), 710.Ql %l 711(local host name), 712.Ql %n 713(host name as provided on the command line), 714.Ql %p 715(remote port), 716.Ql %r 717(remote user name) or 718.Ql %u 719(local user name). 720.Pp 721The command is run synchronously and does not have access to the 722session of the 723.Xr ssh 1 724that spawned it. 725It should not be used for interactive commands. 726.Pp 727This directive is ignored unless 728.Cm PermitLocalCommand 729has been enabled. 730.It Cm LocalForward 731Specifies that a TCP port on the local machine be forwarded over 732the secure channel to the specified host and port from the remote machine. 733The first argument must be 734.Sm off 735.Oo Ar bind_address : Oc Ar port 736.Sm on 737and the second argument must be 738.Ar host : Ns Ar hostport . 739IPv6 addresses can be specified by enclosing addresses in square brackets. 740Multiple forwardings may be specified, and additional forwardings can be 741given on the command line. 742Only the superuser can forward privileged ports. 743By default, the local port is bound in accordance with the 744.Cm GatewayPorts 745setting. 746However, an explicit 747.Ar bind_address 748may be used to bind the connection to a specific address. 749The 750.Ar bind_address 751of 752.Dq localhost 753indicates that the listening port be bound for local use only, while an 754empty address or 755.Sq * 756indicates that the port should be available from all interfaces. 757.It Cm LogLevel 758Gives the verbosity level that is used when logging messages from 759.Xr ssh 1 . 760The possible values are: 761QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 762The default is INFO. 763DEBUG and DEBUG1 are equivalent. 764DEBUG2 and DEBUG3 each specify higher levels of verbose output. 765.It Cm MACs 766Specifies the MAC (message authentication code) algorithms 767in order of preference. 768The MAC algorithm is used in protocol version 2 769for data integrity protection. 770Multiple algorithms must be comma-separated. 771The default is: 772.Bd -literal -offset indent 773hmac-md5,hmac-sha1,umac-64@openssh.com, 774hmac-ripemd160,hmac-sha1-96,hmac-md5-96 775.Ed 776.It Cm NoHostAuthenticationForLocalhost 777This option can be used if the home directory is shared across machines. 778In this case localhost will refer to a different machine on each of 779the machines and the user will get many warnings about changed host keys. 780However, this option disables host authentication for localhost. 781The argument to this keyword must be 782.Dq yes 783or 784.Dq no . 785The default is to check the host key for localhost. 786.It Cm NumberOfPasswordPrompts 787Specifies the number of password prompts before giving up. 788The argument to this keyword must be an integer. 789The default is 3. 790.It Cm PasswordAuthentication 791Specifies whether to use password authentication. 792The argument to this keyword must be 793.Dq yes 794or 795.Dq no . 796The default is 797.Dq yes . 798.It Cm PermitLocalCommand 799Allow local command execution via the 800.Ic LocalCommand 801option or using the 802.Ic !\& Ns Ar command 803escape sequence in 804.Xr ssh 1 . 805The argument must be 806.Dq yes 807or 808.Dq no . 809The default is 810.Dq no . 811.It Cm PKCS11Provider 812Specifies which PKCS#11 provider to use. 813The argument to this keyword is the PKCS#11 shared library 814.Xr ssh 1 815should use to communicate with a PKCS#11 token providing the user's 816private RSA key. 817.It Cm Port 818Specifies the port number to connect on the remote host. 819The default is 22. 820.It Cm PreferredAuthentications 821Specifies the order in which the client should try protocol 2 822authentication methods. 823This allows a client to prefer one method (e.g.\& 824.Cm keyboard-interactive ) 825over another method (e.g.\& 826.Cm password ) . 827The default is: 828.Bd -literal -offset indent 829gssapi-with-mic,hostbased,publickey, 830keyboard-interactive,password 831.Ed 832.It Cm Protocol 833Specifies the protocol versions 834.Xr ssh 1 835should support in order of preference. 836The possible values are 837.Sq 1 838and 839.Sq 2 . 840Multiple versions must be comma-separated. 841When this option is set to 842.Dq 2,1 843.Nm ssh 844will try version 2 and fall back to version 1 845if version 2 is not available. 846The default is 847.Sq 2 . 848.It Cm ProxyCommand 849Specifies the command to use to connect to the server. 850The command 851string extends to the end of the line, and is executed with 852the user's shell. 853In the command string, any occurrence of 854.Ql %h 855will be substituted by the host name to 856connect, 857.Ql %p 858by the port, and 859.Ql %r 860by the remote user name. 861The command can be basically anything, 862and should read from its standard input and write to its standard output. 863It should eventually connect an 864.Xr sshd 8 865server running on some machine, or execute 866.Ic sshd -i 867somewhere. 868Host key management will be done using the 869HostName of the host being connected (defaulting to the name typed by 870the user). 871Setting the command to 872.Dq none 873disables this option entirely. 874Note that 875.Cm CheckHostIP 876is not available for connects with a proxy command. 877.Pp 878This directive is useful in conjunction with 879.Xr nc 1 880and its proxy support. 881For example, the following directive would connect via an HTTP proxy at 882192.0.2.0: 883.Bd -literal -offset 3n 884ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 885.Ed 886.It Cm PubkeyAuthentication 887Specifies whether to try public key authentication. 888The argument to this keyword must be 889.Dq yes 890or 891.Dq no . 892The default is 893.Dq yes . 894This option applies to protocol version 2 only. 895.It Cm RekeyLimit 896Specifies the maximum amount of data that may be transmitted before the 897session key is renegotiated. 898The argument is the number of bytes, with an optional suffix of 899.Sq K , 900.Sq M , 901or 902.Sq G 903to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 904The default is between 905.Sq 1G 906and 907.Sq 4G , 908depending on the cipher. 909This option applies to protocol version 2 only. 910.It Cm RemoteForward 911Specifies that a TCP port on the remote machine be forwarded over 912the secure channel to the specified host and port from the local machine. 913The first argument must be 914.Sm off 915.Oo Ar bind_address : Oc Ar port 916.Sm on 917and the second argument must be 918.Ar host : Ns Ar hostport . 919IPv6 addresses can be specified by enclosing addresses in square brackets. 920Multiple forwardings may be specified, and additional 921forwardings can be given on the command line. 922Privileged ports can be forwarded only when 923logging in as root on the remote machine. 924.Pp 925If the 926.Ar port 927argument is 928.Ql 0 , 929the listen port will be dynamically allocated on the server and reported 930to the client at run time. 931.Pp 932If the 933.Ar bind_address 934is not specified, the default is to only bind to loopback addresses. 935If the 936.Ar bind_address 937is 938.Ql * 939or an empty string, then the forwarding is requested to listen on all 940interfaces. 941Specifying a remote 942.Ar bind_address 943will only succeed if the server's 944.Cm GatewayPorts 945option is enabled (see 946.Xr sshd_config 5 ) . 947.It Cm RhostsRSAAuthentication 948Specifies whether to try rhosts based authentication with RSA host 949authentication. 950The argument must be 951.Dq yes 952or 953.Dq no . 954The default is 955.Dq no . 956This option applies to protocol version 1 only and requires 957.Xr ssh 1 958to be setuid root. 959.It Cm RSAAuthentication 960Specifies whether to try RSA authentication. 961The argument to this keyword must be 962.Dq yes 963or 964.Dq no . 965RSA authentication will only be 966attempted if the identity file exists, or an authentication agent is 967running. 968The default is 969.Dq yes . 970Note that this option applies to protocol version 1 only. 971.It Cm SendEnv 972Specifies what variables from the local 973.Xr environ 7 974should be sent to the server. 975Note that environment passing is only supported for protocol 2. 976The server must also support it, and the server must be configured to 977accept these environment variables. 978Refer to 979.Cm AcceptEnv 980in 981.Xr sshd_config 5 982for how to configure the server. 983Variables are specified by name, which may contain wildcard characters. 984Multiple environment variables may be separated by whitespace or spread 985across multiple 986.Cm SendEnv 987directives. 988The default is not to send any environment variables. 989.Pp 990See 991.Sx PATTERNS 992for more information on patterns. 993.It Cm ServerAliveCountMax 994Sets the number of server alive messages (see below) which may be 995sent without 996.Xr ssh 1 997receiving any messages back from the server. 998If this threshold is reached while server alive messages are being sent, 999ssh will disconnect from the server, terminating the session. 1000It is important to note that the use of server alive messages is very 1001different from 1002.Cm TCPKeepAlive 1003(below). 1004The server alive messages are sent through the encrypted channel 1005and therefore will not be spoofable. 1006The TCP keepalive option enabled by 1007.Cm TCPKeepAlive 1008is spoofable. 1009The server alive mechanism is valuable when the client or 1010server depend on knowing when a connection has become inactive. 1011.Pp 1012The default value is 3. 1013If, for example, 1014.Cm ServerAliveInterval 1015(see below) is set to 15 and 1016.Cm ServerAliveCountMax 1017is left at the default, if the server becomes unresponsive, 1018ssh will disconnect after approximately 45 seconds. 1019This option applies to protocol version 2 only. 1020.It Cm ServerAliveInterval 1021Sets a timeout interval in seconds after which if no data has been received 1022from the server, 1023.Xr ssh 1 1024will send a message through the encrypted 1025channel to request a response from the server. 1026The default 1027is 0, indicating that these messages will not be sent to the server. 1028This option applies to protocol version 2 only. 1029.It Cm StrictHostKeyChecking 1030If this flag is set to 1031.Dq yes , 1032.Xr ssh 1 1033will never automatically add host keys to the 1034.Pa ~/.ssh/known_hosts 1035file, and refuses to connect to hosts whose host key has changed. 1036This provides maximum protection against trojan horse attacks, 1037though it can be annoying when the 1038.Pa /etc/ssh/ssh_known_hosts 1039file is poorly maintained or when connections to new hosts are 1040frequently made. 1041This option forces the user to manually 1042add all new hosts. 1043If this flag is set to 1044.Dq no , 1045ssh will automatically add new host keys to the 1046user known hosts files. 1047If this flag is set to 1048.Dq ask , 1049new host keys 1050will be added to the user known host files only after the user 1051has confirmed that is what they really want to do, and 1052ssh will refuse to connect to hosts whose host key has changed. 1053The host keys of 1054known hosts will be verified automatically in all cases. 1055The argument must be 1056.Dq yes , 1057.Dq no , 1058or 1059.Dq ask . 1060The default is 1061.Dq ask . 1062.It Cm TCPKeepAlive 1063Specifies whether the system should send TCP keepalive messages to the 1064other side. 1065If they are sent, death of the connection or crash of one 1066of the machines will be properly noticed. 1067However, this means that 1068connections will die if the route is down temporarily, and some people 1069find it annoying. 1070.Pp 1071The default is 1072.Dq yes 1073(to send TCP keepalive messages), and the client will notice 1074if the network goes down or the remote host dies. 1075This is important in scripts, and many users want it too. 1076.Pp 1077To disable TCP keepalive messages, the value should be set to 1078.Dq no . 1079.It Cm Tunnel 1080Request 1081.Xr tun 4 1082device forwarding between the client and the server. 1083The argument must be 1084.Dq yes , 1085.Dq point-to-point 1086(layer 3), 1087.Dq ethernet 1088(layer 2), 1089or 1090.Dq no . 1091Specifying 1092.Dq yes 1093requests the default tunnel mode, which is 1094.Dq point-to-point . 1095The default is 1096.Dq no . 1097.It Cm TunnelDevice 1098Specifies the 1099.Xr tun 4 1100devices to open on the client 1101.Pq Ar local_tun 1102and the server 1103.Pq Ar remote_tun . 1104.Pp 1105The argument must be 1106.Sm off 1107.Ar local_tun Op : Ar remote_tun . 1108.Sm on 1109The devices may be specified by numerical ID or the keyword 1110.Dq any , 1111which uses the next available tunnel device. 1112If 1113.Ar remote_tun 1114is not specified, it defaults to 1115.Dq any . 1116The default is 1117.Dq any:any . 1118.It Cm UsePrivilegedPort 1119Specifies whether to use a privileged port for outgoing connections. 1120The argument must be 1121.Dq yes 1122or 1123.Dq no . 1124The default is 1125.Dq no . 1126If set to 1127.Dq yes , 1128.Xr ssh 1 1129must be setuid root. 1130Note that this option must be set to 1131.Dq yes 1132for 1133.Cm RhostsRSAAuthentication 1134with older servers. 1135.It Cm User 1136Specifies the user to log in as. 1137This can be useful when a different user name is used on different machines. 1138This saves the trouble of 1139having to remember to give the user name on the command line. 1140.It Cm UserKnownHostsFile 1141Specifies a file to use for the user 1142host key database instead of 1143.Pa ~/.ssh/known_hosts . 1144.It Cm VerifyHostKeyDNS 1145Specifies whether to verify the remote key using DNS and SSHFP resource 1146records. 1147If this option is set to 1148.Dq yes , 1149the client will implicitly trust keys that match a secure fingerprint 1150from DNS. 1151Insecure fingerprints will be handled as if this option was set to 1152.Dq ask . 1153If this option is set to 1154.Dq ask , 1155information on fingerprint match will be displayed, but the user will still 1156need to confirm new host keys according to the 1157.Cm StrictHostKeyChecking 1158option. 1159The argument must be 1160.Dq yes , 1161.Dq no , 1162or 1163.Dq ask . 1164The default is 1165.Dq no . 1166Note that this option applies to protocol version 2 only. 1167.Pp 1168See also 1169.Sx VERIFYING HOST KEYS 1170in 1171.Xr ssh 1 . 1172.It Cm VersionAddendum 1173Specifies a string to append to the regular version string to identify 1174OS- or site-specific modifications. 1175The default is 1176.Dq FreeBSD-20110503 . 1177.It Cm VisualHostKey 1178If this flag is set to 1179.Dq yes , 1180an ASCII art representation of the remote host key fingerprint is 1181printed in addition to the hex fingerprint string at login and 1182for unknown host keys. 1183If this flag is set to 1184.Dq no , 1185no fingerprint strings are printed at login and 1186only the hex fingerprint string will be printed for unknown host keys. 1187The default is 1188.Dq no . 1189.It Cm XAuthLocation 1190Specifies the full pathname of the 1191.Xr xauth 1 1192program. 1193The default is 1194.Pa /usr/local/bin/xauth . 1195.El 1196.Sh PATTERNS 1197A 1198.Em pattern 1199consists of zero or more non-whitespace characters, 1200.Sq * 1201(a wildcard that matches zero or more characters), 1202or 1203.Sq ?\& 1204(a wildcard that matches exactly one character). 1205For example, to specify a set of declarations for any host in the 1206.Dq .co.uk 1207set of domains, 1208the following pattern could be used: 1209.Pp 1210.Dl Host *.co.uk 1211.Pp 1212The following pattern 1213would match any host in the 192.168.0.[0-9] network range: 1214.Pp 1215.Dl Host 192.168.0.? 1216.Pp 1217A 1218.Em pattern-list 1219is a comma-separated list of patterns. 1220Patterns within pattern-lists may be negated 1221by preceding them with an exclamation mark 1222.Pq Sq !\& . 1223For example, 1224to allow a key to be used from anywhere within an organisation 1225except from the 1226.Dq dialup 1227pool, 1228the following entry (in authorized_keys) could be used: 1229.Pp 1230.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1231.Sh FILES 1232.Bl -tag -width Ds 1233.It Pa ~/.ssh/config 1234This is the per-user configuration file. 1235The format of this file is described above. 1236This file is used by the SSH client. 1237Because of the potential for abuse, this file must have strict permissions: 1238read/write for the user, and not accessible by others. 1239.It Pa /etc/ssh/ssh_config 1240Systemwide configuration file. 1241This file provides defaults for those 1242values that are not specified in the user's configuration file, and 1243for those users who do not have a configuration file. 1244This file must be world-readable. 1245.El 1246.Sh SEE ALSO 1247.Xr ssh 1 1248.Sh AUTHORS 1249OpenSSH is a derivative of the original and free 1250ssh 1.2.12 release by Tatu Ylonen. 1251Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1252Theo de Raadt and Dug Song 1253removed many bugs, re-added newer features and 1254created OpenSSH. 1255Markus Friedl contributed the support for SSH 1256protocol versions 1.5 and 2.0. 1257